1. Field of the Invention
The invention relates to a shunt element for an electrochemical storage cell or a group of several storage cells connected in parallel, of the alkali metal and chalcogen type, with at least one anode space for receiving the anolyte and a cathode space for receiving the catholyte, which are separated from each other by an alkali ion-conducting solid electrolyte wall and, at least in some areas, are bounded by a metallic housing.
2. Description of the Prior Art
Such shunt elements are used primarily in high-temperature batteries composed of electrochemical storage cells of the alkali metal and chalcogen type. The shunt elements electrically shunt destroyed storage cells which can be separated in this manner from the circuit of the high-temperature battery. The operability of the high temperature battery is thereby substantially maintained.
High-temperature batteries which are constructed from electrochemical storage cells, will be used in the future to an increasing extent for the electric propulsion of vehicles.
In such vehicle batteries, it is required that many electrochemical storage cells be connected in series and only a few storage cells in parallel. Reasons for this are that the energy content of such a battery is in general smaller than 40 kWh and the energy content of a single storage cell will be larger than 80 Wh. It follows that a vehicle battery will not contain more than 500 storage cells. If a total of 200 volts is to be generated with such, a battery at a voltage of the individual storage cell of about 2 volts, 100 storage cells must be connected in series. This means that at most 5 storage cells can be connected in parallel. Since, with 5 parallel-connected storage cells the redundancy is not yet very great, it is advantageous to connect as many storage cells as possible in series. Such branches can then be connected in parallel so that the circuit diagram shown in FIG. 1 is obtained. In this drawing, the storage cells are shown only by their electrical terminals. As may be seen from this drawing, n storage cells are connected together in series-fashion to form a branch. m branches with n storage cells each are connected parallel and form a block. The entire battery consists of p such series-connected blocks. According to the example shown in FIG. 1, it contains n.times.m.times.p storage cells.
Problems arise in the described circuit if a storage cell becomes defective. In the case of sodium-sulfur storage cells it has been found that a defect usually occurs by the fact that in time the solid electrolyte develops cracks so that the reaction materials sodium and sulfur can react with each other directly and no voltage is delivered by the storage cell. Such a defective storage cell then has infinitely large resistance which usually is more that a factor 2 greater than the ohmic resistance of an intact storage cell. As the result, less charging and discharging current flows through the branch containing a defective storage cell. If the resistance of the defective storage cell is very high, then practically only m-1 branches of the battery still contribute to the capacity of the battery. This means that the capacity of the entire battery is smaller under these conditions by a factor (m-1)/m than that of an intact battery.